This invention relates to acoustic surface wave devices. More particularly, it is concerned with acoustic surface wave devices employed as filters.
Acoustic surface wave devices employing piezoelectric materials having suitable properties for propagating surface waves and having transducers for launching and receiving acoustic surface waves in the material are well known. Typically, the transducers are arrays of interleaved conductive electrodes deposited on a substrate of the material. In response to electrical signals an input or transmitting transducer launches acoustic surface waves along a predetermined path on the surface of the substrate. An output or receiving transducer detects the acoustic surface waves and generates electrical signals in response thereto. Typically, acoustic surface wave devices have been employed as delay lines and as filters. Because of the frequency response which can be obtained in an acoustic surface wave device by suitably designing the configuration of the transducer electrodes, particularly desirable bandpass characteristics can be achieved such as that required of an intermediate frequency filter for use in television receivers.
In the development of acoustic surface wave devices for use as filters various problems have been encountered. Several secondary effects are present which tend to degrade the performance of the device. Various techniques have been employed to compensate or avoid certain of these secondary effects.
One undesirable secondary effect is known as wave front distortion. In order to obtain the desired frequency response for certain types of filters the electrodes of the input transducer extending in opposite direction from two bus bars are arranged to overlap. With varying overlap the number of metal electrodes traversed by a surface wave moving along its path of propagation varies across the span of the transducer aperture. Since the velocity of acoustic surface waves is affected by traveling under a metallized surface, the result is wavefront distortion. It has been found that the secondary effect can be avoided by the use of so-called "dummy" or inactive electrodes which extend toward each active electrode from the opposite bus bar so as to provide an overall generally rectangular configuration of the transducer. Thus, all acoustic surface waves generated within the overlap region of the transducer aperture traverse essentially the same amount of metallized surface as they pass along the propagation path through the transducer.
Another secondary effect is acoustic reflections caused by impedance discontinuities in the propagating medium. This problem is corrected by the use of dual element electrodes in place of single element electrodes. With single element electrodes the electrodes are generally one-fourth of the principal wavelength wide and adjacent electrodes are generally separated by one-fourth of a wavelength. With the two element electrodes each element is one-eighth of a wavelength wide and adjacent elements are separated by one-eighth of a wavelength. The double element electrode configuration causes undesirable acoustic reflections to cancel each other. This technique is well known and widely used to suppress what is known as triple transit echos.
Another secondary effect is caused by reflections occurring at the edges of the electrodes with either single element or two element electrodes. Although double element electrode structures are efficient in suppressing reflections at the center frequency of the device, this action degrades gradually outward from the center frequency. In many types of acoustic surface wave devices the electrode structure is weighted as to amplitude and phase, that is, the length of the electrodes is varied to vary the overlap and the spacing between the electrodes is varied to produce phase weighting. The problem of reflections from electrode edges may be exaggerated in devices of this type. Although individual edge reflections are small, they can add in phase to significant values to become noticeable spurious signals.
One structure which greatly reduces the problem of spurious signals caused by acoustic reflections from the edges of the electrodes is disclosed and claimed in U.S. Pat. No. 4,205,285 which issued to Martin E. Dempsey and Ching W. Lee on May 27, 1980. In the input transducer the major portions of the inactive electrodes and of the active electrodes which lie outside the overlap envelope and also the bus bars are combined into continuous uninterrupted metal layers. With this structure electrode edges outside of the overlap envelope are reduced significantly, thus greatly reducing the problem of spurious signals caused by acoustic reflections from the edges. However, since the active electrodes within the overlap envelope are designed to generate or detect acoustic surface waves, they cannot be removed or replaced by solid conductive material. Distortion of the desired signal caused by reflection at the edges of the electrodes within the overlap envelope can be significant, particularly with substrates of propagating materials which have strong electrical to mechanical coupling, such as lithium niobate.